In the typical delayed coking process, high boiling petroleum residues are fed to a large vertical vessel, called a coke drum, where they are thermally cracked into light products and a solid residue, petroleum coke. When the drum is full, the feed is diverted to an empty drum. The coke-filled drum is steam purged and cooled with quench water. The drum is then drained of water, and the top and bottom heads are removed to allow the coke to be cut from the drum by high pressure water jets. When the coke is removed, the heads are replaced, and the coke drum is readied to be placed back into service to repeat the cycle.
Conventional coke drum deheading is labor intensive. The massive steel head is typically six or seven feet in diameter and as many as sixty or more bolts may be used to secure it to the drum. A cart or trolley with hydraulically elevated arms has been used to support the bottom head during unbolting and to lower the head after it is unbolted. The cart is also used to raise the head and hold it in place when it is rebolted following the coke removal procedure. It is necessary to move the head out of the way so that a chute or flume can be connected to the bottom of the coke drum to receive the coke/water mixture during coke removal. Working in the vicinity of the bottom head of the coke drum presents a number of hazards, particularly prior to coke removal. Water and other materials can fall out of the drum, because the quench water is not completely drained, or especially in the case of a shot-coke cave-in. A typical coke load can weigh as much as 240,000 pounds, and can easily exceed the capacity of the hydraulic arms and even the floor structure design load.
Various devices are available from the prior art for holding flanges together. While some of these have been very successful in other applications, none have been entirely satisfactory when used in coke drum deheading applications. For example, a sliding C-clamp has been used to hold and pressurize two flanges together. The sliding C-clamp works well when the flanges are broken apart only infrequently, but wear of the clamp due to friction between its pressurizing surfaces and the flanges is a major disadvantage in applications where the flanges are to be broken apart frequently as in coke deheading. As a consequence, the sliding C-clamp has not been used in delayed coking applications as far as applicant is aware.
A number of deheading devices intended specifically for coke drum deheading are known. For example, an article by Nagy et al., Oil and Gas Journal, pp. 77-80, May 29, 1989, describes a bottom head hinged to the coke drum and a pair of hydraulic cylinders used to support the head while operators manually remove the bolts. After the head bolts are removed, the hydraulic cylinders can be remotely activated to swing the head on its hinge to allow access for coke removal. While this approach eliminates the use of the deheading car and achieves some degree of safety by removing the drum head remotely, after the bolts have been removed and operating personnel are away from the drum, there are still a number of drawbacks: the bolts must still be removed manually; supporting brackets for the cylinders are welded directly to the drum shell which can reduce the life of the shell or require additional reinforcement; the hinged connection of the head to the drum prevents maintenance work on the head, i.e. cleaning and gasket replacement, until the decoking operation has been completed and the chute has been removed from the bottom of the drum; and personnel must then service the hinged head under the opened drum from which debris can fall. This device is also disclosed in EP Application No. 89-307,539.
Another type of unheading device using thirty-two swing bolts each equipped with remotely operable detensioning devices is disclosed in U.S. Pat. Nos. 4,960,358 and 4,726,109. In these disclosures, the bottom head is unbolted by disconnecting the swing bolts, lowered on a frame supported from the drum, and moved laterally on a carriage to allow access to the lower flange of the drum. This device uses a large number of moving parts which contribute to complexity and maintenance requirements, requires a specially designed or modified lower flange, head and carriage, and the frame and carriage are supported from the shell of the drum.
A closure device for coke drum deheading operations is also commercially available from Hahn and Clay under the trade designation FACT. With this rather complex device, the conventional bottom head is replaced with a custom head drilled for thirty-two bolts. The head is assembled as a sandwich with another flange or force ring of the same size as the bottom head. The head and force ring are assembled with a series of ramps located along the perimeter of the head. The assembly is held in place by the bolts and high nitrogen pressure is applied to a special force actuator located between the flanges to move them apart. Then a set of three hydraulic cylinders turn a special ramp ring to push the head upwardly from the force ring flange to hold the head in a pressurized position after the nitrogen pressure is removed. Major disadvantages of this device are wear of the ramp ring, problems related to coke accumulation between the flanges and the force ring, its complexity and the use of high pressure friction between components which can result in equipment failure and high maintenance requirements.
Accordingly, there remains a need for a pressure vessel closure device suitable for coke drum deheading which provides automated installation and removal of the drum head, avoids the use of components which are subject to rapid wear and/or high maintenance, and is relatively simple in design.